The present invention relates to a mask read only memory (ROM), particularly to a mask ROM for storing multi-value data.
A mask ROM configured by NOR-type memory cells stores data in the manner of changing threshold voltages of transistors which are configured in the memory cells. The threshold voltages are controlled by an ion injection onto a gate after forming the gate, a source and a drain which serve as word lines. Since such configured memory cells can store only data "0" and "1", it is necessary to provide a number of memory cell transistors corresponding to a storage capacity of a mask ROM. Here, if one memory cell can store data of more than three kinds, it is possible to greatly increase the storage capacity of a mask ROM. In this specification, such a mask ROM is called as a multi-value ROM. For example, if one memory cell can store four kinds data, it is possible to double the storage capacity in one chip without an increase chip area.
A conventional multi-value ROM stores data as more than three kinds, in the manner that changes of channel width, and length of memory cell transistors causes a current drive capability to be changed. Data are written by using a layer mask in forming a thick oxide isolation. However, since an isolation step is positioned as an initial stage of the manufacturing steps of the mask ROM, there is the problem that there is a long turn-around time (hereunder, called TAT), which is a time period from putting user's data into a manufacturing apparatus to a shipment of a product. Accordingly, the conventional multi-value ROM is not practical because data are written by using a layer mask.
Furthermore there are types of conventional multi-value ROMs which store data by changing threshold voltages of memory cell transistors in the manner that a plurality of ion injection stages are performed by using a plurality of masks of different patterns. However, since this type ROM needs a plurality of openings of photoresist as masks, it has the problem that the TAT is long.
As described above, any type of the conventional multi-value ROM has the problem that the TAT is long.